Observatory wall sealants

Forums General Discussion Observatory wall sealants

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    Grant Privett

    I recently set up an old 2.1m Pulsar dome on a concrete base.

    I put No Nonsense external sealant under the walls and hoped that would be sufficient to keep the inside dry, but water was getting in – no surprise, nor panic, as the surface was very rough.

    So, I applied mastic all round the outside. However, after spotting some of the mastic was not adhering well to the gel coated edges of the external wall (it stuck well to the concrete) I peeled some of it off and applied an external window sealer mastic.

    To avoid litigation I won’t say who made it*, but after 2 days of rain the external window sealer had a blistered appearance and appears to have absorbed lots of water and become fragile. In fact, a lot of it wiped straight off and the floor of the dome was wetter than before.

    Given that I would prefer to not use bitumen (but will if I have to), what has everyone else used that they found successful for sealing the outside wall of their observatory to the ground?

    *LATER EDIT: Okay, I have now found lots of derogatory reviews online, so litigation seems unlikely. The mastic that turned to a white  blistered mush after a night’s rain was Everbuild. It looks like it works okay if it sets before it sees rain and is painted it with something waterproof. Left untreated, some people do have serious problems, as I did. When its next dry for 3 days I will try a different sealant.

    Jeremy Shears

    I used EverBuild external frame sealant, Grant.

    I applied both to the outside wall and inside. I overpainted the latter with the polymeric concrete sealant paint base I used for the rest of the floor.

    Grant Privett

    I wonder if my mistake was in not sealing it with paint before the first rain.

    I really had not expected the stuff to absorb the water under any circumstance – not the behaviour you would anticipate from a “sealant”. That sounds more like filler…

    Daryl Dobbs

    Have you tried Gorilla sealant, rain proof in around 4 hours and cures in 24. I’ve found the no nonsense stuff take days of dry weather before it cure and sometimes like yourself it doesn’t.

    so far gorilla hasn’t let me down, or I use Dow 785 which is equally good.

    Daryl Dobbs

    This selection guide might be useful as I’ve used several of their products in various jobs over the years 


    Grant Privett

    Just to clarify, it wasn’t the No Nonsense that failed drastically but a different brand. The No Nonsense sticks to about 75% of the gel coated dome but not all  especially – where a thin coating. I really wonder if roughening with some emery paper or something similar might have provided a better key.

    Thanks for the steer to Gorilla and DOW 785. Have been pondering one of the Soudals too.


    William Scutcher

    The SkyShed Pod supplier recommends Lexel. It  works reasonably well with HD polythene which is more or less a non-stick material for adhesives. I’ve found it very good.  It’s a bit expensive and difficult to find.

    You could try:


    Alan Snook

    I used 3M 4200 marine adhesive sealant when I set up the 3.5m dome (see small ads in the back of JBAA Dec 2019). This was at the recommendation of the dome manufacturer – Sirius in Australia. Get it from some chandlers. The bottom edge of the GRP panels was abraded a little with wet ‘n’ dry to key the surface. I did the work in Sept 2020 and so far it has kept the water out. I got through three tubes of the stuff. However I fear your joint may now be contaminated with mastic, window sealant etc; you will surely need to remove all traces from both the GRP walls and the concrete before using the 3M product or it will be good money after bad,


    The main issue with seaing a Fibre Glass Dome is the flexure of of the material consequently should you nudge against the wall the base will move breaking the seal.

    I used 1 mitre stud lengths cut them intop 6″ lengths using a heavy duty drill inserted around the dome. It is necessary to use a concreate to stainless steal bonding this is inseted into the predrilled hole with the stud length rammed down with a hamer ande finally cap this with a  stailess streal  nut and washer. I have around 20 of these in total. The door needs particular attention.

    Lifting the dome wll as you go around the circumferance of the dome lifing section by section get all the sealent under the rim and bolt down.

    Finally there is a product called “sticks like shit” that works well when mending brken clay pots amongst other things.

    Best of luck


    Grant Privett

    As its water soluble I am hoping the rain over the next couple of days will flush some residue away away and beyond that I am hosing the joint to free up some of the debris left – even if it will wet every thing for a day or two. May have a bash with a cloth soaked with meths or white spirit. Its going to be a week or so until things are dry enough to try bonding again….

    Dr Paul Leyland

    There is stuff here in La Palma called “No mas clavos” made by an outfit named Pattex. Literally translated this is “No more nails”  and a quick web search suggests that it is the same as the UK product.

    Regardless, it sticks almost everything to almost everything, is flexible and waterproof. I used it to get an extra 3 months wear out of a pair of sandals when their soles became unbonded, so I can testify to its strength and flexibility.

    A 250g tube cost only a few Euros a couple of years back.

    Might be worth investigating.

    Added in edit: See https://www.amazon.co.uk/Waterproof-Heavy-Duty-Mounting-Adhesive-Cartridge/dp/B0154WKHGA/ref=sr_1_1?crid=EFMNDM1QNSIX&dchild=1&keywords=no+more+nails+waterproof&qid=1627420009&s=diy&sprefix=no+more+nail%2Cdiy%2C261&sr=1-1

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